The present invention relates to a process for superimposing ray bundles that are emitted by a plurality of individual radiation sources and are combined with the aid of a ray forming and focussing optics system, in which the individual radiation sources are arranged on at least one curve surrounding the optical axis of the ray forming and focusing optics system, as well as an arrangement for superimposing ray bundles that are emitted by a plurality of individual radiation sources, with a ray forming and focusing optics system that combines the ray bundles at a superimposition point, in which the individual radiation sources are arranged on at least one curve surrounding the optical axis of the ray forming and focusing optics system.
Such processes and arrangements are used in cases where the radiation from a plurality of individual radiation sources, e.g. for increasing output, are to be concentrated in a relatively small spatial operating region. Such applications occur in material processing and working, for example in laser welding, cutting or drilling, as well as in surface treatment. Other applications include medical treatment in ophthalmology, in dental technology and in dermatology. A further area of use is in the telecommunications sector, in cases where in a data transmission channel realised by means of an optical fibre, a high light output is required for the information transfer over wide stretches without intermediately-connected amplifiers. A small numerical aperture of the coupled optics system is necessary in such cases.
With such known processes and arrangements laser diode arrays are normally used as light sources, which are termed “bars” or “stacks”. A “bar” comprises a plurality of individual laser diodes arranged in a linear array. A high output laser diode array of this type has a typical light emitted output of about 50W. Typical total emission surfaces of such laser diode a typical light emitted output of about 50W. Typical total emission surfaces of such laser diode arrays have a long side (long axis, so-called “slow axis”) of about 10 mm and a narrow side (short axis, so-called “fast axis”) of ca. 1 micrometre. The individual laser diode structures are in this connection distributed linearly on the long side of the bar (about 10 to 50 pieces) and normally have a width of 40 micrometres to 500 micrometres along the long side of the bar. The beam divergence of the light emitted from the laser diode arrays in the planes parallel to the short axis of the emission surface is typically greater by a factor of 3 to 4 than in the directions perpendicular thereto and thus corresponds to the radiation characteristics of the individual laser diode structures, which are termed “emitters” or “individual emitters”. The radiation from the individual emitters is not coherent with the radiation of the adjacent structures. No light is emitted between the emitters.
Such “bars” can be stacked on one another, resulting in the formation of “stacks”. The emission bundles of the individual emission surfaces of the laser diode arrays are superimposed within the “stack” in order to utilise the light output (optical power) of the whole “stack”. For the this purpose it is known to assign to each individual emitter within the laser diode array stacked in the “stack” a microlens that guides the emission bundle of an individual emitter to the coupling end of an optical fibre associated therewith. In this way a plurality of fibres is associated with a “bar” or also with a “stack”, which can be guided in a fibre bundle. The decoupling end of the fibre bundle can then be imaged in order to generate an operating laser beam bundle. This arrangement has three disadvantages: on the one hand light is lost because the optics system cannot be fully illuminated. On the other hand, in order to obtain a small focal point the focal width of the microlenses must be kept small, which leads to a large numerical aperture associated with coupling losses. Finally, the number of individual light sources that can be added in this way to form a common focal point is limited.
The handling of such a microlens array is relatively complicated since the microlenses have to be arranged close to the individual emitters and the adjustment is correspondingly critical. In addition, when using microlenses in conjunction with high output laser diode arrays the production and choice of material is very problematic since even very small absorptions at the emission wavelength of the laser diodes lead to an unacceptable heating of the microlenses.
Ray guidance systems that concentrate the light from a plurality of spatially separated point-like light sources in a focal point are known. Expressed in wave-optics terms, a distinction is made between processes in which the wave fronts of the individual light sources are used for the overlapping, and those processes in which the wave fronts of the individual light source are aligned next to one another in order to form in the combination a larger overall wave front. In this connection it is assumed that the ray bundle of each individual light source and thus also the wave front, defined as the eikonal area of the ray bundle, has a similar cross-section.
When using the process and the arrangement mentioned in the introduction for laser marking, one of the most important preconditions is that a specific material-dependent threshold of the power density at the workpiece is exceeded by the incident laser radiation. Most surfaces that are to be marked, in particular metals, reflect laser radiation to a large extent. Only after the power density threshold has been exceeded does a capillary (so-called “keyhole”) form in the surface, through which most of the applied laser radiation is instantaneously absorbed. The laser radiation reflected back from the object surface represents a serious problem since this passes back through the focussing optics system, with the result that the optical components that are used for the focussing are subjected to double or in some cases to even greater optical stresses. In this connection not only coatings and glazings but also fibres and crystals in the corresponding configurations may be destroyed. The laser diodes, in particular high output laser diodes, that are used as individual light sources are threatened to an even greater extent. It is known that laser diode systems react very sensitively in the case of back-reflections and can be destroyed by the latter.
Outputs that are not particularly high, normally between 10 and 100 Watts, are required for marking applications. The size and shape of the focal point on the workpiece surface are accordingly very important. Due to the high degree of anisotropy of the radiation that high output laser diodes normally have, as mentioned above, and the lack of coherence of the radiation in the slow axis, it is not possible with normal optical focussing elements to generate, for operating distances customary in practice, a focus that is so small that the aforementioned power density threshold for marking on metallic surfaces is exceeded. By and large high output laser diode arrays are therefore prima facie conceptually unsuitable for such a task on account of their ray quality. Due to the fact that the marking takes place on the surface of the workpieces, in other words unlike in welding or deep welding, a large depth sharpness for the laser focussing is not necessary. With a correspondingly small focal length and large aperture of the focussing optics a sufficiently small focus can thus be generated. However, such a small focus generally utilises nothing in practice since, in particular for marking purposes, a narrow ray bundle, i.e. a small divergence of the rays in the focus, is required in order to be able to deflect the ray bundle via a scanner, generally an XY scanner, which normally has a small free beam aperture.
Most known arrangements for laser marking use lamp-pumped solid state lasers. These have a comparatively poor efficiency. In flash lamp-pumped solid state lasers the pumping energy is derived from the emission spectrum of the flash lamps. The overall efficiency of such arrangements is ca. 3%. The use of laser diodes for pumping solid state lasers considerably increases the efficiency of the solid state lasers pumped in this way (ca. 50% for the high output laser diodes, ca. 50% for the optical coupling of the energy and, finally, a further ca. 50% efficiency of the solid state laser itself, which corresponds to an overall efficiency of at best 12.5%). It is therefore easily understandable why the direct use of laser diodes with an efficiency of about 50% is economically so attractive.
Known focussing arrangements that employ exclusively laser diodes are often not very effective, even though the efficiency of the laser diodes is very high. For example, arrangements are known in which a ray bundle is deflected at a partly light-transparent deflection mirror so that it runs in parallel to and in an overlapping manner with a second ray bundle. A loss-free coupling is possible if the deflection mirror has a wavelength-dependent reflectivity and the two ray bundles have different wavelengths. In a similar way a polarising beam splitter cube can combine two ray bundles with orthogonal polarisation. The disadvantage is that only two ray bundles can be combined.
It is also known to combine a plurality of ray bundles of different wavelengths by means of a diffraction grating. The disadvantage is that the coupling efficiency of the diffraction grating is far less than 100% and monochromatic light sources with an accurately-graded sequence of wavelengths are required.
In known arrangements of the type mentioned in the introduction an optical element (lens, mirror, prism) is in each case associated with a linearly-arranged group of light sources of the laser diode structure or with an individual light source, the object of which optical element is to deflect or form the light of this group of light sources so that the focal points of the ray bundles that derive from all individual light sources of the laser diode structure come together at one point.
An example of such an arrangement is described in WO 99/64912 A1. In this case a two-dimensional matrix of microlenses is used, each of which collimates the ray bundle of an individual light source. After the microlens matrix the rays of all individual light sources run in parallel, so that a focussing lens can collect the ray bundles in a single focal point. In a variant of this known principle the microlens matrix is replaced by two line-type systems of cylindrical lenses arranged one after the other. The cylindrical axis of the first system are perpendicular to the cylindrical axes of the second system. A line or gap of light sources is associated with each cylindrical lens. The cylindrical lenses of the first system align all rays parallel to a plane, and the cylindrical lenses of the second system align the rays parallel to one another.
The arrangement described in DE 195 37 265 C1 uses a matrix of microprisms that deflect the ray bundles of a matrix of individual light sources of a laser diode structure so that all ray bundles apparently have the same starting point. An optical system arranged there behind forms an image of the common virtual object point on the focal point in the focal plane.
In a further group of known arrangements, such as are described for example in DE 195 14 626 A and DE 197 25 262 A, pairs of stepped mirrors or stepped prisms are used that displace the individual ray bundles of a system of ray bundles running in parallel but widely spaced apart from one another, in such a way that the distance between adjacent ray bundles is reduced or brought to zero.
The disadvantage with all these known arrangements is that, as already indicated above, a diode bar contains 12 to 48 individual emitters arranged in lines and an optical deflection element of the described type has to be produced for each individual emitter. In arrangements that cannot be adjusted, during manufacture position tolerances of the deflection elements have to be maintained in the micrometre range so that the ray bundles can be sufficiently accurately aligned in parallel. With arrangements that can be adjusted, each of these 12 to 48 deflection elements must be positioned to micrometre accuracy. Furthermore, the permanent stability of the adjusted positions must be guaranteed.
Further processes for combining the wave fronts of individual light sources are described in DE 100 62 453 A, DE 100 62 454 A and DE 199 49 198 A. Here, annular arrangements of diode bars that ensure a homogeneous illumination of the focal point are employed. The beam quality is adversely affected by the use of diode arrays; due to the admittedly technologically advantageous free optical axis of the focussing head the beam quality is impaired by reduction of the “filling factor”. Due to the fact that the ray-forming components are arranged around the central free optical axis, a greater divergence of the ray bundle in the focus is produced than when using the central space.
In U.S. Pat. No. 6,137,631 A the light from a laser diode bar is collimated with the aid of a cylindrical lens and a following spherical optics system. The opening through which the light enters a reflector element, consisting of a pair of parallel mirrors, is located at the point of intersection of the ray bundles of the individual emitters. The ray bundles are reflected variously often depending on the angle of incidence. A focussing lens that generates an intermediate image of the bar is located at the outlet opening of the reflector element. The arrangement is similar to that described in DE 101 21 678 A. Although this arrangement can handle all images of the individual emitters of a bar very efficiently, the focal point is however only the enlarged image of one individual emitter and the arrangement is restricted for practical reasons to one diode bar. The beam quality is however improved by increasing the so-called “degree of filling”, since the individual ray bundles from different directions arrive at the focal point.
Finally, from U.S. Pat. No. 4,826,269 A, a circular laser diode arrangement is also known, in which an anamorphotic image, which is disadvantageous in many respects, is used to focus the laser diodes.